Tricolor cathode ray tube having porous graphite layer on aluminum screen coating



J. P. DRIFFORT ET AL TRICOLOR CATHODE RAY TUBE HAVING POROUS GRAPHI LAYER ON ALUMINUM SCREEN COATING Filed Nov. 27, 1967 Oct. 28, 1969 A a g-n United States Patent US. Cl. 313106 3 Claims ABSTRACT OF THE DISCLOSURE A three-gun, tricolor picture tube of the type including a post-focusing grid adjacent the luminescent screen. The screen is provided with an aluminum coating upon which there is deposited a porous graphite layer having a density between 0.2 and 0.3 and a thickness between 13 and 17 microns.

This invention relates to color television image tubes which include at least one post-accelerating and postfocusing grid located near the screen.

In tubes of this kind, a high-intensity electric field is developed between the post-focusing grid and the screen, and the latter is bombarded by high-energy electrons (energies in the order of 20 to 25 kev.). The impact of the electrons, which is responsible for the luminescence effect, also gives rise to the undesirable phenomenon of secondary electron emission. The secondary electrons, knocked out of the surface of the screen, leave that surface at various angles, are deflected back (due to the opposed high-intensity electric field prevailing in the space between screen and grid), return to the screen in the neighborhood of their points of origin, and thus produce a halo effect around the spot produced on the screen by the electron beam proper.

The image definition thus deteriorates and the background of the screen becomes brighter so that the contrast is weaker. In addition, in color television, the color fidelity and saturation are impaired since the secondary electrons knocked out of the luminescent elements of each of the three primary colors fall back onto elements of any color.

Furthermore, the luminescent elements of the screen are generally covered by a very thin aluminum layer, and a small fraction of the primary electrons arriving at the screen are reflected either by the luminescent layer or by the aluminum. These reflected primary electrons act like the secondary electrons and thus also contribute to the development of the halo.

It is well known that the halo can be suppressed or attenuated to some extent by providing the screen with a decelerating layer which absorbs a major part of the energy of the reflected or secondary electrons. To this end, a light material is used which has a low coefficient of secodnary electron emission and a high thermal energy evacuation factor, such as graphite.

However, a layer of this kind also reduces the energy of the primary electrons and therefore inevitably reduces the image brilliance.

Studies which have been carried out in the applicants laboratories have shown that a decelerating layer can be produced which will absorb the great majority of the secondary electrons and reflected electrons, thereby considerably increasing the contrast while reducing the brilliance level only to a very'small extent.

Accordingly, it is the main object of this invention to provide an improved color television picture tube for ob- 3,475,639 Patented Oct. 28, 1969 ice taining images with a highly increased contrast in comparison with those of the prior art.

It is another object of the invention to suppress or substantially attenuate the halo effect in picture tubes of the type described.

In accordance with the invention there is provided a three-gun, tricolor picture tube having a luminescent screen and a post-focusing grid adjacent the screen, said screen being provided with a thin coating and a decelerating graphite layer deposited on the aluminum coating, wherein the graphite layer is porous and has a density d and a thickness e such that the product d.e is comprised between 2.6 and 5.1, d being expressed in gr./cm. and e in nucrons.

In a preferred embodiment of the invention the density of the porous graphite layer is comprised between 0.2 and 0.3, while the thickness thereof is comprised between 13 and 17 microns.

An embodiment of the invention will now be considered with reference to the single figure of the attached drawing. The figure illustrates in section the application of the improvement in accordance with the invention to a tricolor image tube with three electron guns.

The improved part of the tube includes a post-focusing grid, composed of wires such as 1, 1a, 2, 2a, and a luminescent screen 3 having a plane surface parallel to the grid.

The screen 3 comprises, in the order listed: a porous powdered graphite layer 4 between 13 and 17 microns in thickness; a high-reflectivity metal layer 5, preferably aluminum, vaporised on under vacuum and having a thickness of about 5 microns; and tricolor luminescent strips 6, 7, 8, 7 8 etc. arranged on a flat glass plate 9.

The electron beams 10, 11, 12 strike the respective strips 6, 7, 8 due to the focusing effect of the screen-grid arrangement.

The porous graphite layer 4 is applied to the aluminum layer 5 by spraying on a colloidal suspension of graphite grains in ethyl alcohol. The grain diamteer is in the order of 1 micron. To this suspension, a fihn-forming product, which may be a colloidal or a methyl methacrylate, is added. The film-forming product is eliminated subsequently during the normal heat treatment phases in the process of manufacture of the screen, and the carbon combustion residues of this product ensure cohesion between the grains themselves and between the grains and the layer 5.

Spraying is effected using a suitable spray gun, and in this phase of the process the contexture of the deposit can be controlled to a large extent by choosing or adjusting the various parameters involved: (a) size of the gun nozzle; (b) nature of the fluid (air, nitrogen, argon, etc.); (c) fluid pressure; (d) size of the graphite grains; (e) concentration of graphite in the suspension; (f) nature of the liquid in which the graphite is in suspension, and of any additional products which may modify the physical and chemical characteristics of the suspension.

The thickness of the graphite layer can be adjusted with great accuracy by suitably fixing the duration of the spraying operation for a constant flow of fluid to the spray gun.

The Applicants have determined experimentally that the product brilliancexcontrast reaches a maximum when the product thicknessxdensity of the graphite layer has a value between 260 and 510 micrograms/cmfi.

Within these limits, the brilliance increases as the density of the graphite layer reduces, but layers having a density below 0.2 gr./cm. do not generally have sufficient uniformity and stability. It has been found that the best results are obtained with a density between 0.2 and 0.3, and a thickness between 13 and 17 microns.

Since, in the compact state, the density of graphite is about 2.3, the range of densities from 0.2 to 0.3 corresponds in terms of the ratio of volumetric occupation by the porous graphite to a range between 9 and 13%. It is within this range that the porous graphite layer gives the best results.

In this way, it has been possible to achieve contrasts as high as 150 or even 200, whereas with screens produced in accordance with the prior art technique the contrast was in the order of 20 to 50.

We claim:

1. A color TV picture tube including a tricolor luminescent screen, three electron guns each adapted to emit an electron beam toward said screen, and a postfocusing grid located on the path of said electron beams adjacent said screen, said screen being provided with a thin aluminum coating on the side facing said electron guns and a porous graphite layer deposited on said aluminum coating, the density d, whose value lies between 0.2 and 0.3 grams per cubic centimeter, and the thickness e in microns of said porous graphite layer being 4 such that the product d.e is comprised between 2.6 and 5.1.

2. A picture tube as claimed in claim 1, wherein the thickness thereof is comprised between 13 and 17 microns.

5 3. A picture tube as claimed in claim 2, wherein the porous graphite layer is composed of grains having a diameter in the order of 1 micron.

10 References Cited UNITED STATES PATENTS 2,858,466 10/1958 Sternglass et al 313107 2,878,411 3/1959 Alvarez 313-107 X 15 2,916,664 12/1959 Sternglass 313-106 X 3,223,872 12/1965 Raibourn.

ROBERT SEGAL, Primary Examiner U.S. Cl. X.R. 

